Method of sintering nickel-bronze articles



United States Patent US. Cl. 75-201 2 Claims ABSTRACT OF THE DISCLOSURE Sintered nickel-bronze articles, such as bearings, made from a mixture of elemental powders containing, by weight, 7% to 8% tin, 4.5% to 5.5% nickel, 0.25% to 4% graphite, balance copper, are characterized by low dimensional sensitivity to variations in sintering temperature.

This invention is directed to a novel mixture of elemental powders for producing sintered nickel-bronze articles from compacts having a low dimensional sensitivity to variations in sintering temperature.

A Well-known process in the prior art, involving the production of bearings and structural parts composed of bronze alloys by powder metallurgy techniques, requires that copper and tin powders mixed with a small proportion of graphite, pore-forming agents and die lubricants, be compacted in suitable dies to form green compacts which are subsequently sintered. The sintering temperature employed is sufficiently high to cause the tin to melt and thereafter dilfuse rapidly into the still solid copper constituent to form the desired bronze.

The powder mixture most commonly used contains approximately 90% copper and 10% tin and is dimensionally highly sensitive to changes in the temperature and time employed for sintering. The sintering furnace temperature will vary somewhat from time to time within the normal tolerance of the furnace temperature controls and through erroneous settings and from other causes. Aside from such variations in average furnace temperature, there may be local overheating or underheating due to such factors as placement of heating elements and location and volume of the furnace charge.

Quite small changes in furnace conditions can lead to a wide variation in the size of products sintered from substantially identical green compacts. The result is that an undesirably high proportion of scrap units is produced.

It has been suggested, in order to increase the tensile strength of sintered bronze bearings and other products, to employ a powder mixture containing from 1% to tin, from 1% to 15 carbonyl nickel powder and from 0.25% to 4% graphite, the balance, apart from impurities, being copper, with or Without pore-forming agents and die lubricants. Sintered alloys of the suggested composition do exhibit relatively good tensile strength. However, the reproducibility of the sintering behavior of such compositions under commercial conditions has proved to be generally unsatisfactory.

A critical range of powder metal constituents has now been discovered for the production of nickel-bronze articles by powder metallurgical techniques which, during the sintering of green compacts, are relatively insensitive to variations in sintering temperatures and times.

It is an object of this invention to provide a mixture of elemental powders having a composition suitable for producing sintered nickel-bronze bearings and similar articles.

It is a further object of this invention to provide constituent powders for nickel-bronze bearings and similar articles which, during sintering, have substantial dimensional stability over a relatively wide range of sintering temperatures.

Other objects and advantages of the invention will, in part, be obvious and will, in part, become apparent here-' inafter.

In accordance with the invention, the desired characteristics are obtained using a mixture of the powder constituents tin, nickel, graphite and copper in a relatively narrow and critical range of composition. This range of composition is, by weight, from 7% to 8% tin, from 4.5% to 5.5% nickel, from 0.25% to 4% graphite, and the balance essentially copper with small amounts of incidental impurities.

An optimized mixture composition for the elemental powders within the above-defined range contains, by weight, about 7.5% tin, about 5% nickel, about 1% graphite and the balance essentially copper except for small amounts of incidental impurities.

With powder compositions of the above-defined range and optimum composition, the variability of dimensional change in percent is relatively low with variations in sintering temperatures over the range 760 C. to 830 C.

The copper constituent of the powder employed in making the compacts of the invention may be in the form of a powder substantially all of which will pass through a 100 mesh (British Standard Screen) sieve. The tin constituent is preferably in the form of a powder substantially all of which will pass through a 300 mesh (B.S.S.) sieve. The nickel constituent is preferably added in the form of fine carbonyl nickel powder (average particle size 9 microns). The graphite constituent may Ibe added as a fine powder that will pass through a 300 mesh B.S.S. sieve.

The powder mixture may contain pore-forming agents or die lubricants or both, for example, a metallic stearate such as zinc stearate or paraffin wax.

The constituent powders are first thoroughly mixed and tumbled for 2 hours in a double cone mixer.

The powders are then formed into hearings or other articles by the usual process of pressing, sintering and sizing or coining, and the invention includes such sintered articles.

For the purpose of giving those skilled in the art a better understanding of the invention, the following illustrative example is given.

EXAMPLE Green compacts were made employing the optimized powder mixture composition of, by weight, 7.5 tin, 5% nickel, 1% graphite and the balance copper except for small amounts of incidental impurities. In pressing the mixed powder for making sintered articles, approximately grams of the mixed powder were pressed at 8 tons per square inch in a 1" diameter steel die. This compaction step yields green compacts with final densities of 5.8 to 6.2 grams per cubic centimeter. The green compacts were 1" in diameter and approximately 1" high.

The compacts were sintered at temperatures of 760 C., 780 C., 800 C., or 830 C. for times of 15, 30, 45 or 60 minutes in a cracked ammonia atmosphere. The sintering time was measured as the time at the given temperature after an initial heating period of 15 minutes to bring the compact to the sintering temperature. Cooling was accomplished in a water-jacketed cold zone of the furnace. In the following Table I, the value for dimensional change during sintering was determined along a diameter of the cylinder of two compacts sintered at each temperature, and the mean of the two values was taken. In addition, in Table I, the percentages of nickel and tin are specifically set forth; it being understood that the mix- :ure contained 1% graphite, and 0.75% zinc stearate with :he balance copper and impurities.

TABLE I The figure in the last column of Table I is the diiIerence bet-ween the extreme values of dimensional change over the test sintering temperature range used, and is an index of the sensitivity of the dimensions of the compacts to :hanges in the sintering temperature. In the case of'the optimized composition of Table I, this value is only 0.15%. It should be noted that the actual amount of dimensional change at each temperature is relatively small, and that there is the further advantage that the dimensional change is always in the same direction (positive) over the range of temperature studied.

In order to show the criticality of the powder mixture composition of the invention, sintering tests essentially identical to those described above were carried out on a wide range of bronze compositions, both with and without nickel. The results obtained in these tests are set forth below in Table 11.

TABLE II Dimensional change (percent) Variability of As in Table I, the figures in the last column of Table II are the difierences between the extreme values of dimensional change, and constitute an index of the sensitivity of the dimensions of the compact to changes in sintering temperature.

These results show clearly the great variability of dimensional change in the standard 90% copper-10% tin, nickel-free bronze (Mixture No. 1). The addition of 2.5% nickel worsens this condition (Mixture No. 5). Reduction in the amount of tin in the powder composition does reduce the variability of dimensional change (Mixture N0. 2), but to a substantially lesser extent than is achieved with the optimized composition of Table I (Mixture A). Most of the compositions of Table II have the disadvantages that they exhibit both positive and negative dimensional change over the test temperature range.

The composition according to the invention is also relatively insensitive to sintering time, though this is of lesser significance under commercial conditions, since the sintering time can be more readily controlled. The dimen- 4 sional changes of compacts of Mixture A after sintering for diiferent times at 800 C. are set forth in Table III.

TABLE III Radial dimensional Sintering time (min.): change (percent) Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. In a method for making sintered nickel-bronze alloy members by powder metallurgy techniques which comprises pressing the constituent powders to produce green compacts of predetermined configuration and thereafter sintering the green compacts to produce a1- loy members from the compacted constituent powders in the predetermined configuration, the improvement comprising controlling the composition of the constituent powders from which the green compacts are made to a relatively restricted range to minimize the dimensional sensitivity of the compacts to variations in the sintering temperature, the constituent powders consisting of, by weight, from 7% to 8% tin, from 4.5% to 5.5% nickel, from 0.25% to 4% graphite and the balance essentially copper except for small amounts of incidental impurities and exhibiting low dimensional sensitivity to variations in sintering temperature over the temperature range 760 C. to 830 C.

2. A method in accordance with claim 1 wherein the constituent powders consist of, by weight, about 7.5% tin, about 5% nickel, about 1% graphite and the balance essentially copper except for small amounts of incidental impurities.

References Cited UNITED STATES PATENTS 1,154,701 9/1915 Kruger 205 X 3,000,734 9/1961 Grant 29-l82.5 X 3,275,426 9/1966 Rowady 75202 X FOREIGN PATENTS 807,093 1/1959 Great Britain.

BENJAMIN R. PADGE'IT, Primary Examiner.

A. I. STEINER, Assistant Examiner.

US. Cl. X.R. 291 82.5; 75214 

